As a general method for producing a bonded wafer, there are known a method wherein a silicon wafer provided with an oxide film (insulating film) is bonded to another silicon wafer and one of the bonded wafers is subjected to grinding/polishing to form an SOI layer (grinding and polishing method), a method wherein oxygen ions are implanted into an inside of a silicon wafer and then annealing at a high temperature is conducted to form a buried oxide film in the inside of the silicon wafer to thereby render a top portion of the oxide film into an SOI layer (SIMOX), a method wherein hydrogen ions or the like are implanted into a surface layer portion of a silicon wafer corresponding to an SOI layer side (wafer for active layer) to form an ion implanted layer, and a silicon wafer for support substrate is bonded thereto, and then exfoliated at the ion implanted layer through a heat treatment to form an SOI layer (Smart Cut method) and the like.
Among the above methods, however, the grinding and polishing method has a problem that the thickness uniformity of the active layer is poor (not less than ±30%). Also, in the method using oxygen ion implantation (SIMOX), there is a problem that it is impossible to produce SOI structures of different crystal orientations with an insulating layer sandwiched therebetween.
In order to solve the above problems, the inventors have already developed a process combining the oxygen ion implanting method and the grinding and polishing method, namely,
“A method for producing a bonded wafer by directly bonding a wafer for active layer having or not having an insulating film on its surface to a wafer for support layer and then thinning the wafer for active layer, which comprises a time-series combination of:
a step of implanting oxygen ions into the wafer for active layer to form an oxygen ion implanted layer in the active layer;
a step of subjecting the wafer for active layer to a heat treatment at a temperature of not lower than 1100° C. in a non-oxidizing atmosphere;
a step of bonding the wafer for active layer to a wafer for support layer;
a step of heat-treating for improving a bonding strength;
a step of grinding a portion of the wafer for active layer in the bonded wafer short of the oxygen ion implanted layer;
a step of further polishing or etching the wafer for active layer to expose the oxygen ion implanted layer;
a step of oxidizing the bonded wafer to form an oxide film on the exposed surface of the oxygen ion implanted layer;
a step of removing the oxide film; and a step of heat-treating at a temperature of not higher than 1100° C. in a non-oxidizing atmosphere.”, which is disclosed in Patent Document 1.
According to the method disclosed in Patent Document 1, it has become possible to provide a direct bonded wafer, which is excellent in the thickness uniformity of the active layer and relatively less in the defects as evaluated with a transmission electron microscope (TEM).
Patent Document 1: JP-2006-184237